In many vapor-phase chemical processes, it is necessary to precisely meter the flow rate of chemicals as vapor-phase constituents of a flowing gas medium. For example, in the fabrication of semiconductors, hydrogen or nitrogen as a carrier gas can be bubbled through a liquid vapor source such as silicon tetrachloride to pick up vapors of the silicon tetrachloride and carry the vapors into a reaction chamber. In the reaction chamber, a portion of the silicon tetrachloride can be made to pyrophorically react to form a pure silicon deposit in the form of an epitaxial layer on a semiconductor wafer. Since the rate of deposition is related to the amount of silicon tetrachloride present in the reaction chamber, one must accurately control the amount of silicon tetrachloride picked up by the carrier gas. It is also desirable to know the absolute flow rate of the silicon tetrachloride so that the results may be correlated and, in some applications, so that the entire process can be automated.
Regulation of vapor mass flow rate is accomplished by utilizing one of several techniques. One method meters the source liquid to a flash vaporizer but is not practical for extremely small flow rates, particularly of corrosive chemicals because of the lack of metering precision and the effects of contamination. In another method, carrier gas is conducted through a bubbler to pick up vaporized source liquid. The rate of source vapor pick-up is controlled by simultaneously regulating parameters such as the pressure, temperature and volume flow rate of the carrier gas, the temperature and pressure of the source liquid in the bubbler, and the temperature of the various plumbing lines to the extent that conditions are regulated and the degree of saturation of vapor and carrier is constant. This method often has inadequate sensitivity for some combinations and does not give an indication of absolute mass flow rate but requires judgments to be made on a trial-and-error basis with performance judged by testing the product after deposition is completed.
Another, more recent method uses a bubbler and a combination of thermal conductivity cells and mass flow sensors. The ratio of source vapor flow rate to carrier flow rate is measured by thermal conductivity analysis of the carrier gas as it enters the bubbler and of the mixture as it emerges from the bubbler. The mass flow of the carrier gas is also measured prior to entering the bubbler. The flow rate of the carrier gas and the above ratio are electronically multiplied to yield the rate of flow of the source vapor. Details of this method are disclosed in C.F. Drexel U.S. Pat. No. 3,650,151. The method overcomes drawbacks of the other methods and permits the absolute mass flow rate of a fluid to be controlled and monitored regardless of changes in pressures, temperatures, levels or other variables. However, the method is limited by the accuracy of the thermal conductivity measurement. Additionally, because the thermal conductivity cells are heated, the vapor is exposed to the possibility of contamination from the cells themselves. Also, long term build-up of vapor material on the walls of the thermal conductivity cells can result in subtle changes in volume of the cells, requiring periodic recalibration. Such recalibration involves the use of standarized mixtures and is thus not only time consuming but expensive.
The present invention provides a system for accurately monitoring and controlling the absolute mass flow rate of a fluid in a fashion similar to the Drexel method, that is regardless of changes in pressures, temperatures, levels or any other variables, but which eliminates the complexity and inaccuracy associated with thermal conductivity cells. Additionally, it provides a means for rapid self-calibration, which is accomplished with such a degree of ease that the system can be recalibrated before every run, thereby greatly enhancing the reliability of the system. In accordance with the present invention, a pair of mass flow sensors are used to monitor the mass flow rates of a carrier gas and of a mixture of the carrier gas and a source vapor formed at a mixing station. The rates are compared, for example, by electronically subtracting the carrier gas flow rate from the mixture flow rate, to generate a signal representing the mass flow rate of the source vapor. The flow of the carrier gas to the mixing station is modulated in accordance with the difference between this signal and a command signal representing a predetermined flow rate. As a result of the modulation, accomplished by appropriately throttling the valve in the carrier gas line, as changes in temperature or pressure vary the pick-up rate, i.e., the concentration, of the source vapor, the system automatically readjusts the carrier flow to maintain the vapor flow rate at the desired level.
More specifically, a control system is provided for accurately metering the amount of a predetermined liquid vaporized by a carrier gas and includes a mixing station, a flow controller, a flowmeter and various signal comparing and generating means. The mixing station can be a bubbler as known to the art or any mechanism for mixing carrier gas with vaporized liquid. The flow controller comprises an inlet for carrier gas, and an outlet, a mass flow sensor connected between the inlet and outlet which serves to generate a signal in correspondence to the flow rate of the carrier gas through the sensor, and an adjustable value which controls flow through the outlet to deliver carrier gas to the mixing station. The flowmeter comprises an inlet for receiving the mixture from the mixing station, and an outlet therefor, and a second mass flow sensor connected between the flowmeter inlet and outlet which serves to generate a signal in correspondence to the mass flow rate of the mixture. An electronic signal from the flow controller mass flow sensor is compared to an electronic signal from the flowmeter mass flow sensor, e.g. by subtracting the first signal from the second signal, to yield a third signal representing the mass flow of the vaporized liquid. A command signal generated in accordance with a predetermined mass flow rate of vaporized liquid is compared to the third signal and any difference therebetween is used to adjust the flow controller valve so as to minimize or eliminate any such difference.
Since the vapors used in semiconductor chemical processes are generally of a contaminating nature, any component exposed to the vapor is prone to develop contamination build-up which adversely affects the calibration accuracy over a long period of time. A significant advantage of the present invention is that calibration can be verified and readjusted as necessary prior to each process run without removal of components from the system and without requiring special calibration equipment to assure repeatable results. Specifically, the carrier gas is diverted from the outlet of the flow controller to the inlet of the flowmeter, bypassng the mixing station. Accordingly, in a calibration step, only carrier gas flows through the flowmeter mass flow sensor. Therefore, the output of the flowmeter mass flow sensor can be electronically adjusted to precisely match the output of the flow controller mass flow sensor giving a resultant source indication of zero. This capability significantly lengthens the service life over other types of instruments used in this type of application. Furthermore, almost any gas can be used as the carrier for almost any chemical vapor. This allows for many new carrier/chemical combinations that could not previously be handled by a thermal conductivity cell instrument, for example, due to inadequate sensitivity.